1. Field of the Invention
This invention lies in the field of washing coal with water only in a shallow bottomed centrifugal separating cyclone of circular cross-section having a cylindrical portion with a diameter to a height ratio of 0.8 to 1.3, preferably 0.90 to 0.95, the cyclone fitted with a single inlet pipe, a shallow dish below the cylindrical portion, a single bottom orifice fitted to the shallow dish and a fixed vortex finder leading to an outlet at the top for removal of washed coal. Gravity separation under streamlined flow is accomplished with light coal particles at a gravity value down to about 1.3 using crushed coal ranging in size from 13/4".times.0 down to 3/8".times.0.
The invention also lies in the field of providing an easily insertable abrasion resistant bottom dish having unique toughness and wear resistance characteristics to provide trouble-free, efficient coal washing based on the special material characteristics and the critical geometry of the shallow bottom dish which adapts it to fit in a closely contoured relationship to the cylindrical portion of the cyclone.
Further, the invention lies in the field of rebuilding cyclones to include the insertable dish and orifice and set the critical adjustments of the invention.
The invention also lies in the field of cleaning ores other than coal to rid them of impurities by taking advantage of the newly discovered efficiency and capacity taught in the present application.
2. Description of the Prior Art
1. Copending Application, Ser. No. 860,330, filed Dec. 14, 1978:
My copending application, Case No. 1, Ser. No. 860,330, filed Dec. 14, 1977, is incorporated herein by reference and teaches creating directed streamlined flow by directing incoming high solids concentrations of crushed coal in water tangentially along the wall of the bowl of a shallow bottomed cyclone while diverging two streams, namely the incoming inlet coal slurry stream and the swirling coal slurry stream, in the cyclone. As a result, the essential preliminary condition of streamlined flow is created. This flow must occur in the centrifugal cyclone in order to accomplish efficient and high capacity washing of coal or other ores to separate impurities having a different gravity than the cleaned material.
2. Prior literature on Operation of Centrifugal Separating Cyclones:
Chemical Engineers' Handbook by Robert H. Perry and Cecil H. Chilton, published by McGraw-Hill Book Company, at pages section 21 page 57 describes the operating conditions for the separating cyclone water washing of coal, namely inlet pressure of about 10 to 14 pounds per square inch gauge pressure for a 20 to 24 inch cyclone, which is the commonly used cyclone size in coal washing plants. The lower limit below which recovery of low gravity coal cannot be achieved is about 6 to 8 pounds per square inch gauge pressure. Finer sizes of crushed coal are separated at slightly higher pressures but pressues above 14 pounds per square inch are not recommended because of accelerated wear. Residence time is very short. The cyclone shown in the Handbook has a long cone and a large volume is circulated for each ton of feed treated in the cone. This results in high energy consumption, low tonnage recovery based on water used and high equipment cost.
Coal Processing Equipment of Uniontown, Pa. describes a Var-A-Wall coal washing plant in the brochure entitled "Hydronomic Modular, Multimedia Coal Washer". The Coal Processing Equipment plant is designed to provide an outside adjustable wall to increase the height of the cyclone. The dominant feature is jigging with washing done under low water pressure. The extension of the cylinder wall length and volume and a variable depth adjustment of the vortex finder tube create a higher energy loss in a longer cyclone with greater water requirements.
The Keystone Coal Industry Manual, Copyright 1977, McGraw-Hill, Inc., is a directory of mechanical coal cleaning plants which describes the name, location, daily capacity, type of cleaning and plant design. The directory identifies 175 plants within the continental United States and 2 plants in Canada which use low pressure jigging cyclones for coal washing at low solids. Most of these jigging cyclones are heavy media plants utilizing a magnetite suspension. Substantially all heavy media cyclones operate at recommended 10 to 12 pounds per square inch pressure. Present recommendations to coal plant operators is to utilize jigging action and steeper cones so that the pressure drops in the cone substantially to atmospheric pressure at the refuse outlet.
The article "Preparation Trends" published in World Coal, Mar. 1978, page 13, gives the basic performance data for a heavy media jigging cyclone (24 inch). The crushed coal feed is 3/8".times.0 which is separated in three fractions, e.g., 3/8".times.28 mesh, 28 mesh .times.100 mesh and 100 mesh .times.0. These plants operate at a 1.76 density separation. Magnetite losses are about 1 kilogram per ton of coal washed. The objective is for a separation as low as 1.40 relative density.
The Jan. 1, 1978 issue of Coal Age, pages 65 through 84, provides a portfolio of flow sheets for the washing plant at the American Electric Power Mine, Helper Site, Salt Lake City, Utah using heavy media cyclones and special water conservation methods. A similar heavy media plant is shown of the Roberts and Schaefer design with a production rate of 1,750 tons per hour. A third heavy media plant from McNally-Pittsburgh is shown for the Jefferson County Mine in Alabama. Still another heavy media Heyl and Patterson cyclone plant is shown which is designed for existing 650 Mw generating units. Yet another preparation plant is shown in Mingo County, W.Va. All of these use heavy media and all are in the multi-million dollar category. In contrast, the capital investment in the present retrofitted cyclone is a small fraction of these costs. To illustrate, the McNally-Pittsburgh plant at Wilson, Md. invested 96 million dollars to process 1,000 tons per hour by jigging while the two stage plant of the invention invests slightly less than 1 million dollars to process 150 tons per hour by streamlined centrifugal separation. At the same output, the jigging choice costs 15 times as much as the centrifugal separation of the invention.
Fitch, U.S. Pat. No. 2,981,413, dated Apr., 1961, proposed the use of a vortex finder as a classifier means in a large capacity cyclone for the separation of fine from coarse particles in a process of separating solids in liquid suspension.
Visman, U.S. Pat. No. Re. 26,720, dated Nov., 1969, was the first to realize success in keeping size separation, as in Fitch, to a minimum while achieving gravity separation using finely crushed coals. Visman's examples are all at 1/4".times.0 at low pulp solids at about 10% in contrast to 10% to 35% of solids herein. Visman's object was to achieve a jigging action along a horizontal section of his uniquely designed cyclone to separate fine particles from coarse particles in contrast to centrifugal separation herein. Both Visman and Fitch first created turbulence by jigging and then tried to control turbulence at the separation zone where the light particles were removed from the heavy particles. In contrast, the invention herein described avoids turbulence.
Loughner, U.S. Pat. No. 3,887,456, dated June, 1975, discloses a shallow bottomed separating cyclone in which controlled turbulence by jigging is introduced into the bowl by riffler means. In Loughner, rifflers are provided to gently open a bed of heavier particles and release lighter particles, thereby permitting the lighter particles to be displaced and more centrally aligned for more complete separation.
Samson et al, U.S. Pat. No. 2,377,524, dated June, 1945, is cited by Fitch in his U.S. Pat. No. 2,981,413, as an early example of an unobstructed freely whirling liquid in the interior of the casing having an axis of radial symmetry, the casing fitted with a vortex finder for clean particles at the top and an orifice at the bottom through which the heavy particles of grit and sand are removed. Samson emphasized the high velocity of 25 feet per second which sets up centrifugal separating forces to push heavy particles against the wall of the cone creating a vortical whirl which causes an upward stream of lights at the center of the cone. Both Fitch and Samson teach a long cone dimension, in Samson 5 to 15 times the diameter of the cylindrical portion, leading one away from the shallow dish concept of the present invention.
Hirsch, U.S. Pat. No. 2,975,896, dated March, 1961, describes the basic construction of a three piece cyclone, e.g., a top cylindrical portion bolted to an intermediate conical portion which is in turn bolted to a bottom tapered orifice portion. Hirsch recognized that the tapered dish constituting the intermediate portion and the orifice portion would wear faster, necessitating replacement of the worn part.